The present invention relates to copiers, facsimiles and printers, more particularly to an image forming apparatus and an image forming method for spraying toner to a recording member for carrying out recording.
Large amounts of documents have recently begun to be treated in offices in accordance with improvement in the performance of personal computers. Furthermore, printers and copiers having high processing capabilities come into wide use in accordance with progress in network technology. In addition, colored documents tend to increase in number in accordance with the abrupt widespread use of ink jet and the like printers. However, apparatuses capable of outputting monochrome documents and colored documents in satisfactory conditions, e.g. at high speed, are now in the course of development, and the advent of such apparatuses is waited for at present.
Prior art will be described below referring to the accompanying drawings. FIG. 8 is a schematic sectional view showing an electric signal recording apparatus disclosed in the Japanese published examined patent application, Publication No. Sho 44-26333.
Referring to FIG. 8, toner 75 is negatively charged for example by friction with fur at the rotation of a brush 76. A power source 77 is connected across a mesh electrode 74 and a back electrode 71 to generate an electric field for accelerating the charged toner 75 toward a recording member 72.
When an electric signal 78 responding to image information is applied across the mesh electrode 74 and a control grid 73 in the above-mentioned electric field generation condition, the amount of the toner 75 passing through the mesh electrode 74 and the adhesion position of the toner 75 to the recording member 72 can be controlled responding to the polarity and intensity of the applied electric signal 78. A parallel electric field for accelerating the toner 75 toward the recording member 72 is generated between the mesh electrode 74 and the back electrode 71 by the power source 77.
When an "on" signal (a positive signal in case the toner 75 is negatively charged) is used as the electric signal 78 and applied to the control grid 73, a gate is opened, and the toner 75 moves toward the recording member 72 along the parallel electric field. On the other hand, when an "off" signal (a negative signal in case the toner 75 is negatively charged) is used as the electric signal 78 and applied to the control grid 73, the gate is closed, and the toner 75 cannot pass through the control grid 73. In this way, images are recorded in accordance with the combination of the "on" and "off" signals used as the electric signal 78.
FIG. 9 is a sectional view showing a schematic configuration of another image recording apparatus disclosed in the Japanese published examined patent application, Publication No. Hei 2-52260. Referring to FIG. 9, a signal electrode 101 and a base electrode 103 are formed around a hole 104 with an insulator 102 disposed therebetween. A recording member 105, e.g. a paper, moves on the surface of a back electrode 106 disposed opposite to the signal electrode 101. The back electrode 106 is connected to a DC power source 109, and a voltage of about 300 V is applied to the back electrode 106. One-component insulating magnetic toner 111 is transferred from a toner transfer member 107 to an image forming position.
An AC power source 108 is connected across the toner transfer member 107 and the base electrode 103. A signal source 110 is connected to the signal electrode 101 and the base electrode 103, and a recording voltage of 50 V is applied to the electrodes 101 and 103. Referring to FIG. 9, the toner transfer member 107 is provided with a stationary magnet 112, and a magnetic blade 114 is provided in the vicinity of the toner transfer member 107.
Next, the image forming operation in the conventional image forming apparatus shown in FIG. 9 will be described below. Thin layers of the one-component insulating magnetic toner 111 are formed on the toner transfer member 107 by the magnetic blade 114. When an AC signal on which an AC or DC signal is superimposed is applied across the base electrode 103 and the toner transfer member 107, the one-component insulating magnetic toner 111 begins a reciprocating or vibrating motion. When a recording signal is input to the signal electrode 101, the one-component insulating magnetic toner 111 passes through the hole 104, and adheres to the recording member 105 in accordance with an electric field applied to the back electrode 106.
As a result, an image is formed on the recording member 105. On the other hand, when no voltage is applied to the signal voltage 101, or when a voltage with the opposite polarity is applied, the one-component insulating magnetic toner 111 does not pass through the hole 104, and no image is formed.
However, the above-mentioned conventional image recording apparatuses have the following problems.
(1) First, a problem is described below, which is encountered in a conventional image forming apparatus disclosed in the Japanese published examined patent application, Publication No. Sho 44-26333 and shown in FIG. 8, wherein the parallel electric field generated between the mesh electrode 74 and the back electrode 71 is controlled by an electric signal applied to the control grid 73. In the case of this kind of conventional image forming apparatus, a sufficient distance is required to be provided between the mesh electrode 74 and the control grid 73 so that the open/close signal generated by the control grid 73 functions sufficiently. However, if a large distance is provided between the mesh electrode 74 and the control grid 73, the toner 75 to be sprinkled is less controllable, whereby proper images cannot be obtained.
Furthermore, in the case of this kind of image forming apparatus, the toner 75 is sprinkled around the apparatus, whereby the basic performance of the apparatus is not satisfactory. Conversely, if the mesh electrode 74 is disposed so as to be close to or make contact with the control grid 73, signals having a large voltage difference must be used as electric signals. This requires the use of a switching device suited for high-voltage switching. For this reason, it is inevitable that this kind of image forming apparatus becomes larger in size and higher in cost.
(2) In such a system disclosed in the Japanese published examined patent application, Publication No. Hei 2-52260 and shown in FIG. 9, wherein the signal electrode 101 and the base electrode 103 are formed on both sides of the insulating member 102, the lines of electric forces for jumping the toner 111 are intensely generated between the signal electrode 101 and the base electrode 103. For this reason, the toner 111 is liable to adhere to an inner wall surface forming the hole 104, and the hole 104 is liable to be clogged with the toner
(3) Furthermore, in the system wherein the signal electrode 101 and the base electrode 103 are formed on both sides of the insulating member 102, the lines of electric forces are generated between the two electrodes at all times. Therefore, the toner 111 is liable to adhere to the inner wall surface of the hole 104. This requires another means for eliminating the toner 111 adhered to the hole 104.